The electrical potential distribution on the external surface of concrete can be indicative of various ongoing processes of interest. For example, surface potential mapping is often conducted to identify the location of corroding reinforcing steel. For that application, potential measurements are normally conducted by connecting the positive terminal of a high-impedance direct current voltmeter to the reinforcing steel assembly embedded in the concrete and the negative terminal to a reference electrode, typically a copper-copper sulfate electrode (CSE). The tip of the reference electrode is then placed in successive contact with an array of points on the external concrete surface and the potential for each point is recorded. The resulting potential map provides a diagnostic of the presence and position of regions of the reinforcement assembly with high likelihood of active corrosion.
The above-described method relies on the sizable potential transition (e.g., from about −150 millivolts (mV) to about −400 mV CSE) that formerly passive steel often experiences upon the onset of active corrosion. Thus, regions displaying negative potentials on the order of several hundred millivolts versus CSE may be considered suspect of active corrosion. If active corrosion affects only a portion of the steel, the potential is not uniformly highly negative on the concrete surface because of the finite resistivity of concrete that joins the active and the passive portions of the assembly. The steel in the latter portion is only partially polarized and potential measurements against nearby concrete remain only mildly negative, so the external potential map can reveal the location of the corroding zones as well. Because of measurement artifacts, the measured potentials may be more or less globally offset from those in an ideal case. Thus, identification of corroding regions often relies more on consideration of potential gradients rather than of the absolute potential values.
Concrete surface potential measurements are also conducted to perform corrosion rate measurements. Those measurements are polarization measurements in which the amount of impressed current needed to achieve a small potential change is determined. The current is impressed by means of an additional external electrode attached to the concrete surface or by means of rebar not in metallic contact with that being tested. Within certain limitations, the ratio of potential change to impressed current density yields the polarization resistance Rp, which is related to the corrosion current density icorr through the Stearn-Geary parameter B by the equation icorr=B/Rp. The resulting value of icorr can then be related to the corrosion rate of the steel by the usual Faradaic conversion.
Surface potential measurements are sensitive to the condition of both the bulk of the concrete and its surface. For example, the presence of a carbonated concrete skin, even if it is very thin, can result in an appreciable potential difference (e.g., as much as 200 mV) between the outer surface and the bulk of the concrete. That difference reflects the widely different pH of pore water in the outer and inner regions. A diffusion potential (a general term that includes junction and membrane potentials as well as those resulting from other electrokinetic effects) develops to preserve charge neutrality upon coupled diffusion, across the region joining both zones, of anions (OH−) and cations (K+, Na+) that have significantly different diffusivities. Weathering, sulfate attack, and other environmental interactions may cause similar electrochemical potential gradients that may affect the surface potential pattern. A very dry concrete surface may prevent accurate potential determination as the effective contact resistance begins to approach the value of the voltmeter input impedance. The sensitivity of the potential measurements to these phenomena creates both an opportunity for their characterization and a concern as a source of artifacts in the corrosion condition determination.
The extent to which potential measurement artifacts are present is obscured by the disruptive nature of the electrochemical reference electrode, which requires a shared electrolyte link between the metallic terminal and the concrete pore water. When the electrolyte tip of the reference electrode touches the concrete surface, a liquid transport process begins that transfers some of the electrode solution into the concrete pores and vice versa. The process may range from mostly interdiffusion if the concrete pores are nearly saturated, to strong convective capillary action if the pores are nearly dry. The latter case may result in appreciable drift in the voltmeter reading as the system slowly approaches a steady-state condition, likely involving the evolution of a diffusion potential pattern that includes both junction and membrane potential components. Such drift can introduce added uncertainty to the result of the potential measurement and significant artifacts in electrochemical corrosion rate measurements. A pre-wetting procedure is sometimes used for the concrete surface before placing the reference electrode to partially alleviate these effects, but comparable uncertainty exists as to the potential variation (and its time variation) created by the intrusion of the wetting fluid to the formerly dry concrete.
In addition to the above-described drawbacks of conventional potential measurement techniques, the process of conditioning (wetting) the concrete can be time consuming and labor intensive. In most cases, many (e.g., hundreds of) wet sponges must be applied to the concrete to be tested and often must be left in place for extended periods of time to achieve the level of saturation necessary to perform measurements.
In view of the foregoing discussion, it can be appreciated that it would be desirable to have an alternative system or method for assessing reinforced concrete.